Fundamentals of magnetic measurement

As magnets and assemblies arrive at your facility from magnet manufacturers, quality control engineers measure to confirm the performance of the unit's magnetic characteristics. Testing might involve several procedures that use the results of magnetic field measurement, including:

Sorting sub-assemblies

Confirming magnetic field characteristics versus applied current

Mapping a magnetic field shape for a component or assembly

Measuring fringe fields or residual fields

Diagnosing detrimental effects of an external field

Measuring the leakage of magnetic field around a shipping container

Measuring operator exposure to magnetic fields, where local or federal regulations apply

Proper use of magnetic testing throughout the fabrication process will help assure that the final, assembled product or system will perform as intended.

Magnetic Units of Measure

Measuring magnets requires a basic understanding of the common units of measure and techniques for characterizing magnetic fields. If you’re not accustomed to working with magnets, terms such as tesla, gauss, and oersted can seem quite alien. Even more confusing is the industry’s use of more than one standard of measurement — cgs and SI. While many engineers in the U.S. typically use cgs, SI is the system of choice for scientists and engineers in the global community. Until there is clear consensus on using one system or the other, it is helpful for technicians and engineers to know how to use both.

Certain magnetic units are used in industrial applications because of their convenience or correlation to a particular application. A few common cgs and SI units and conversions QC engineers will come across are shown in Figure 1.

Quantity

cgs

SI

Flux

Ø

maxwell (Mx)

weber (W)

Flux density

B

gauss (G)

tesla (T)

Magnetic field strength

H

oersted (Oe)

A/m

Magnetic dipole moment

m

emu

Wm & Am2

Permeability

µ

H/m

1 weber = 108 maxwell

1 tesla = 10,000 gauss

1 oersted = 79.6 A/m

(Wcm) × (4π × 10-5) = Am2

1 milligauss = 0.1 microtesla = 100 nanotesla

1 millitesla = 0.001 tesla = 10 gauss

1 gamma = 0.01 milligauss = 1 nanotesla

Figure 1. Units used to measure magnetism

For those new to magnetic measurement, it is helpful to first consider magnetic flux, generally denoted as Ø. The basic component of flux, expressed in terms of Maxell (Mx) or Weber (W). The amount of this flux per unit area, or flux density, is denoted as B and expressed in terms of gauss (G) or tesla (T) values. This is the field component measured naturally by a Hall sensor based on a teslameter/gaussmeter. Flux density (B) is related to the magnetic field strength (H) which is measured naturally with a fluxmeter.

Although these two instrument types measure slightly different parameters of a magnetic field, it is possible to convert between them with the following relationship:

B = µH

This equation provides the basis for additionally measuring magnetic field strength (H) in air (µ is a known constant) with the Hall effect teslameter. Permeability (µ) is a measure of the ease with which a magnetic field flows in a medium.

Measuring magnetic fields requires specialized sensors and a knowledge of physics and electronics. You can use a variety of instruments, including teslameters, fluxmeters, and magnetometers, to measure magnetism.

No matter which instrument you use, proper technique is important to ensure accurate results. Learn how to prevent mistakes by downloading the whitepaper, Top 5 Most Common Sources of Error in Magnetic Measurement.